Various data storage apparatuses for storing secret data in a nonvolatile storage medium have been proposed.
For example, in order to restrict the reading of the data stored, Japanese Unexamined Patent Publication No. 107352/1987 (Tokukaisho 62-107352; published on May 18, 1987) discloses an “encryption ROM device” which includes a ROM for storing data, and a volatile memory or a volatile register into which an encryption key is written. In the case when data is stored in the encryption ROM device, the encryption key is written into the volatile memory or the volatile register, and data is encrypted with the encryption key to store in the ROM. Meanwhile, in the case when data is read out from the encryption ROM device, the encryption key is written into the volatile memory or the volatile register, and data in the ROM is decrypted with the enciyplion key. The encryption key written into the volatile memory or the volatile register is easily erased by turning off the power of the encryption ROM device, and it is arranged so that data stored in the ROM cannot be read out only by turning on the power again.
Moreover, as to the secret data which is not needed any more, Japanese Unexamined Patent Publication No. 223061/1997 (Tokukaihei 9-223061; published on Aug. 26, 1997) discloses “information processing device” in which upon completion of the reading of the secret data, not only an index including FAT which manages data storage locations but also secret data are erased from the information storage area, such as a hard disk storing secret data, for enhancement of confidentiality.
However, in the “encryption ROM device” of Japanese Unexamined Patent Publication No. 107352/1987, there is the possibility that when the content of the ROM storing easily analyzable data is analyzed by the outsider who has smuggled it out, the encryption key that has been erased might be guessed. There is the danger that when the encryption key was known by non-interested persons, they might sequentially decrypt the data stored in the encryption ROM device. This threatens not only other data stored in the analyzed ROM, but also data that will be stored in the encryption ROM device in the future.
Moreover, in case of the “information processing device” of Japanese Unexamined Patent Publication No. 223061/1997, secret data itself, in addition to the index, is erased, so that the data to be erased has a large volume. Consequently, the time required for erasing increases, which might result in the decrease in efficiency of data input and output with respect to the information storage area.